The present invention deals with disc drives. More specifically, the present invention deals with controlling an electromechanical actuator for positioning a transducer over a disc in a disc drive.
A typical disc drive includes one or more magnetic discs mounted for rotation on a hub or spindle. A typical disc drive also includes one or more transducers supported by a hydrodynamic air bearing which flies above each magnetic disc. The transducers and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls a disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs.
An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored on the magnetic discs by providing a write signal to the data head to encode flux reversals on the surface of the magnetic disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the magnetic disc, sensing the flux reversals on the magnetic disc, and generating a read signal based on those flux reversals. The read signal is then decoded by the drive controller to recover the data represented by flux reversals stored on the magnetic disc, and consequently represented in the read signal provided by the data head.
In performing a track seek operation, the disc drive controller causes a voltage to be applied across the electromechanical actuator in a first polarity causing the electromechanical actuator to accelerate in positioning the transducer over the disc. As the data head approaches the desired track (or target track) on the disc, the disc drive controller causes voltage from the power supply to be applied across the electromechanical actuator in an opposite polarity in order to decelerate the electromechanical actuator, and eventually stop the electromechanical actuator over the desired track. Characteristics of each disc drive can vary from disc drive-to-disc drive, and during the life of, or even during a single operational period, of a disc drive. For example, the actual output voltage provided by the voltage supplies in each disc drive can vary within certain tolerances, as can the magnet strength of the magnets in each electromechanical actuator. Further, as the disc drive operates, it typically heats up. Temperature variations in the disc drive can also affect optimum actuator control. Finally, as a drive ages, it can undergo changes caused by signal values drifting and components fatiguing.
In order to accommodate these variations in drive characteristics, prior systems control the electromechanical actuator according to a deceleration curve which assumes a worst case scenario. In other words, the deceleration schedule for decelerating the electromechanical actuator during a seek operation is based on the weakest magnets expected for production, the highest temperature expected during drive operation, and the lowest power supply voltage expected in any given drive. Controlling the drive according to this deceleration schedule amounts to assuming that only a certain level of current was available to the electromechanical actuator in order to decelerate the electromechanical actuator. In actuality, the current available to decelerate the electromechanical actuator is significantly greater than that which is assumed. Therefore, the deceleration (and hence the seek operation) has taken longer in prior drives than necessary.